Violectric HPA V281 User Manual page 27

Conductive-plastic resistive tracks, high-quality multi-tap wipers and
separated chambers for the individual sections are highly desirable for
sophisticated applications, and high quality is inevitable to ensure
trouble-free operation for years. Since the market for really good pots is
a small one, manufacturers like Noble or Panasonic don't offer these
any more. A current sample of top of the line pots is the RK27 by ALPS,
which is also used in your HPA V281.
Why is a low output impedance essential ?
When actuated, electro-dynamic systems respond with a counterforce.
When the voice coil of a headphone has been displaced by the signal,
an (error-) current will be induced when it swings back to its initial
position. This current must be suppressed as far as possible, which is
effected best if the amplifier's output impedance is the lowest possible.
The damping factor describes nothing but the ratio between output
impedance of an amplifier and a given load.
Since there is no known technical specification, we define the load
(voice coil impedance) as 50 ohms. With V281 having an output
impedance of <0.2 ohms in balanced mode and <0.1 ohms in
unbalanced mode this results in a damping factor of 250 (balanced) and
damping factor of 500 (unbalanced).
Why are high supply voltages essential ?
A headphone doesn't really require high power, but from the equation
P = U2 / R we can see that the square of the supply voltage determines
the power into a given load resistance. The higher the headphone's
impedance, the more voltage will be needed. But this deals with the
achievable loudness to a limited extent only: Technically spoken, music
lives on fast transients which put high demands on signal processing.
And thus a fast transient can easily push an average amplifier with +/-15
volts supply to its limits (90 % of all headphone amps in the market are
operated with these or even lower supply voltages). Due to the high
supply voltage and the balanced operation mode of V281 you will
benefit from more than 4 x output voltage swing capability compared to
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